The present invention relates to an electric machine.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
An electric machine of a type involved here has a stator and an armature, with the armature including as the magnetically active part permanent magnets as well as magnetically soft flux conducting elements arranged between them. The magnetically active part of the armature is the constituent which is involved to a significant extent in the actual magnetic interaction between the stator and armature via the air gap of the electric machine. To be distinguished therefrom is the part of the armature that simply supports the magnetically active part and serves to mechanically transmit forces, i.e. in the case of a rotor for example its shaft.
In order for an electric machine to generate sufficient torque within a given installation space during operation of the motor, attempts have been made to provide a greatest possible magnetic flux density in the region of the air gap between the stator and armature of the electric machine. In the case of electric vehicles, for example, for a drive motor it is necessary to provide a flux density measuring an average of 0.6 to 0.8 tesla in the air gap. Same considerations apply, when a generator is involved with reference to generating an electric voltage.
In the case of permanently excited armatures, i.e. armatures that are not electrically energized via coils and the magnetic field of the armature is produced solely via permanent magnets, adequate flux density can be produced via a suitably large number of permanent magnets. This requires the provision of a large number of poles in the armature. That in turn causes a high remagnetization frequency in the armature and stator as a result of fast-changing stator fields, e.g. at high rotational speeds. When a drive motor for an electric vehicle is involved, the rotor should be able to rotate in the range of 5000 to 10000 revolutions per minute. That can cause substantial eddy currents and hence cause the electric machine to become very hot.
It would be desirable and advantageous to provide an improved electric machine to obviate prior art shortcomings and to enable transmission of a large torque, even when rotational speeds are high.